The friction stir welding method is a solid-phase joining method, and thus has an advantage that deformation or cracking caused by thermal strain is less likely to arise as compared with an arc welding process, such as TIG or MIG. Also, when the arc welding process is employed for joining workpieces of a light metal, such as aluminum, in such a manner as to have a weld penetration depth of 5 mm or greater, multi-layer welding is required. Thus, the number of steps increases. Also, due to occurrence of a blowhole defect or hot cracking, there arises a problem of deterioration in joining quality. Meanwhile, even in the case of a weld penetration depth of 5 mm or greater, an electron beam welding process can achieve joining by a single pass. However, since electron beam welding must be carried out within a vacuum, apparatus cost increases. Furthermore, since the beam diameter is small, the dimensional accuracy of beveling and positioning accuracy must be enhanced. Thus, associated setup work is rather troublesome. By contrast, in joining with a joint depth of up to about 25 mm, the friction stir welding method can readily achieve joining by a single pass without involvement of the above-mentioned defects involved in arc welding. Also, since consumables, such as filler metal and shield gas, are not used, implementation cost is low.
However, even in the friction stir welding method, in the case of a long joint, a deep joint, workpieces having high deformation resistance, etc., the amount of frictional heat generated during joining is significantly large. Accordingly, in the case of time-consuming joining, joining of workpieces having high deformation resistance, joining of workpieces having a shape susceptible to accumulation of heat, etc., a temperature difference becomes large between a joining start zone and a joining end zone, thereby raising a problem of great variation in joining quality along the longitudinal direction of the joint portion.
A proposed friction stir welding method which has solved the above problems employs cooling means which are disposed on opposite sides of a friction-stir-welding tool and which move synchronously with the friction-stir-welding tool (refer to Patent Document 1). In friction stir welding, during movement of a probe of the friction-stir-welding tool and the cooling means, the cooling means spray a cooling medium over a leading zone and a trailing zone with respect to the direction of movement of the probe, and a control unit controls a spraying rate.
However, according to the method disclosed in Patent Document 1, the cooling means must be moved synchronously with the friction-stir-welding tool, and the spraying rate of the cooling medium from the cooling means must be controlled by the control unit, thus involving a problem of an increase in cost of apparatus for carrying out the method. Also, since the cooling medium is sprayed over a leading zone with respect to the direction of movement of the probe, the cooling medium may penetrate into an unjoined interface between workpieces, and, in this condition, joining may be carried out. Therefore, joining quality may deteriorate. Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. 2004-148350